The present invention relates to torque control of variable reluctance motors and deals more specifically with a method and related commutation apparatus for controlling torque and smoothing torque ripple produced by a variable reluctance motor.
It is generally desirable to use variable reluctance motors in positioning systems because they can be used as direct drive motors to produce rotation in small, discrete steps. In addition, variable reluctance motors are generally low cost, small in size and have a high torque-to-inertia ratio. In spite of their desirability for use as servomotors, variable reluctance motors are limited in their application due to torque ripple and nonlinear torque-to-input current ratio. Torque ripple is generally defined as the variation in the maximum available output torque as the position of the rotor varies with respect to the stator.
Known techniques for controlling torque and reducing torque ripple have not proved to be totally satisfactory. One approach attempts to produce constant torque by modulating the current supplied to the motor wherein the current is limited during the high torque portion of the cycle. This approach has the disadvantage of limiting the maximum torque developed by the motor to a level which can be substantially below the peak torque.
Another known technique is to energize more than one phase in a multiphase motor during those portions in the rotor's rotation where the torque developed by the individual phases is near its minimum. This approach reduces the torque to about 80% of peak torque and generally requires a more complex commutation control circuit than otherwise used to control the energization of the stator voltagne phases.
It is an object of the present invention therefore to provide a method and related commutation apparatus for controlling a variable reluctance motor to produce a constant desired torque without torque ripple.